JP2023136503A - Image forming apparatus - Google Patents

Abstract

To prevent variations in electrification voltage associated with displacement of a surface potential of a photoconductor drum due to pre-electrification exposure.SOLUTION: An image forming apparatus comprises process cartridges 225y, 225m, 225c, and 225k each having a photoconductor drum 215, an electrifying roller 216 that electrifies a surface of the photoconductor drum 215, and a pre-electrification exposure device 227 that has a light-emitting device 301 and irradiates the photoconductor drum 215 with light emitted from the light-emitting device 301 to eliminate static electricity from the surface of the photoconductor drum 215. When a voltage generation circuit 601 applies electrification voltage to electrifying rollers 216 of the process cartridges 225y, 225m, and 225c, a voltage generation circuit 602 applies electrification voltage to the electrifying roller 216 of the process cartridge 225k. A CPU 209 changes the quantity of light emitted by the light-emitting device 301 from a first light quantity to a second light quantity having a larger quantity of light than the first light quantity, and changes from the second light quantity to a third light quantity having a larger quantity of light than the second light quantity.SELECTED DRAWING: Figure 7

Description

The present invention relates to an image forming apparatus using an electrophotographic method, such as a copying machine or a printer.

In image forming devices that use electrophotography, such as copying machines and printers, an electrostatic latent image is formed on a photosensitive drum that has been charged to a uniform potential by a charging device by irradiating light according to image data. . Then, by applying toner, which is a developer, from a developing means to the electrostatic latent image formed on the photosensitive drum, the electrostatic latent image is visualized, and a toner image is formed. Thereafter, the toner image formed on the photosensitive drum is transferred to a recording material such as a recording paper by a transfer means, and the toner image is fixed to the recording material by a fixing device, thereby forming a desired image.

In a color image forming apparatus, toner images of different colors are formed on a photosensitive drum in each image forming section provided for each toner color, and the formed toner images of different colors are sequentially superimposed on the same recording material. A color image is formed by being transferred together. Therefore, generally, a charging device that charges the photosensitive drum of each image forming section to a uniform potential and a power supply device that supplies power to the charging device of each image forming section are required. As one means for realizing miniaturization and cost reduction of color image forming apparatuses, for example, Patent Document 1 proposes a technique of supplying power to a plurality of charging devices from a common power supply device.

Furthermore, from the viewpoint of downsizing image forming apparatuses and eliminating waste, a cleanerless system (toner recycling system) has been proposed, for example, in Patent Document 2. The cleaner-less system is configured to eliminate a dedicated drum cleaner, which is a cleaning means for removing toner remaining on the photosensitive drum after the transfer process of transferring the toner image to the recording material. In a cleanerless system, the residual toner on the photosensitive drum after the transfer process is removed from the photosensitive drum by cleaning and removing it from the photosensitive drum at the same time as development by the developing means ("cleaning at the same time as development"). It can be collected and reused.

Transferred residual toner is mainly toner that is charged to a positive polarity that is opposite to the normal negative polarity, or toner that is charged to the normal negative polarity but does not have sufficient charge. Such as toner. In the configuration of a cleanerless system that performs cleaning at the same time as development, it is necessary to prevent residual toner from being transferred from adhering to a negatively charged charging roller. Therefore, for example, in the image forming apparatus of Patent Document 3, the contact portion where the photosensitive drum contacts the charging roller is located downstream of the contact portion where the photosensitive drum and the transfer roller contact with respect to the rotational direction of the photosensitive drum. A pre-charging exposure device is installed upstream of the charging section. The pre-charging exposure device is a device that eliminates the surface potential of the photosensitive drum by irradiating it with light (hereinafter referred to as "pre-charging exposure").The pre-charging exposure device eliminates the static potential of the photosensitive drum by irradiating it with light, and eliminates the potential difference between the photosensitive drum and the charging roller. Make it bigger. As a result, discharge occurs at the contact portion between the photosensitive drum and the charging roller, making it possible to uniformly charge the residual toner to a negative polarity. As a result, in an image forming apparatus equipped with a cleaner-less system, it is possible to suppress adhesion of transfer residual toner to the charging roller.

In addition, if there is a residual charge on the photosensitive drum, the surface potential of the photosensitive drum will be in a disordered state, so especially in a low humidity environment, the rotation of the photosensitive drum may occur due to the potential difference between the charged potentials on the photosensitive drum. An image defect called drum ghost may occur at regular intervals. In order to suppress drum ghosts, for example, in Patent Document 4, after the transfer process and before the charging process using the charging roller, light is irradiated onto the surface of the photosensitive drum to bring the surface potential of the photosensitive drum to a predetermined residual potential level. A pre-charging exposure device has been proposed that eliminates static electricity. Furthermore, for example, in Patent Document 5, the surface potential of the photosensitive drum in the rotational axis direction after static elimination is uniformed by guiding the light irradiated onto the photosensitive drum through a light guide such as a light guide during pre-charging exposure. Static elimination configurations have been proposed.

Japanese Patent Application Publication No. 2016-126252 Japanese Patent Application Publication No. 2006-301108 Japanese Patent Application Publication No. 2019-174765 Japanese Patent Application Publication No. 2001-142365 JP 2017-58433 Publication

However, in the structure of a cleanerless system, when performing pre-charging exposure using a pre-charging exposure device, in order to suppress the adhesion of transfer residual toner to the charging roller and the occurrence of drum ghosts, there is It is necessary to irradiate the light accordingly. FIG. 11 is a timing chart showing the states of the pre-charging exposure device, the photosensitive drum, and the charging voltage when the color image forming apparatus is in printing operation. In FIG. 11, 101y, 101m, 101c, and 101k indicate the amount of light emitted from the light-emitting elements of the pre-charging exposure device provided in each image forming section with different toner colors. 102y, 102m, 102c, and 102k indicate the transition of the surface potential near the exposed portion of the photosensitive drum irradiated with light by the pre-charging exposure device of each image forming portion. Reference numeral 103 indicates the output waveform of the charging voltage output from the power supply device. Further, 104y, 104m, 104c, and 104k indicate the timing at which the image forming area on the photosensitive drum that is irradiated with the laser light according to the image data of each image forming section passes through the contact portion with the charging roller. There is.

When pre-charging exposure is performed by the pre-charging exposure device, the surface potential of the photosensitive drum before and after the pre-charging exposure changes sharply from -300V to -150V, as shown at 102y, 102m, 102c, and 102k in FIG. . In the following, a steep change in the surface potential of the photosensitive drum before and after the pre-charging exposure is referred to as "load fluctuation." Therefore, when the irradiation part irradiated with light by the pre-charging exposure device on the photosensitive drum reaches a part close to the charging roller, the high voltage power supply that supplies power to the charging roller will load the photosensitive drum. It is no longer possible to follow the fluctuations, and the output of the charging voltage fluctuates. As a result, the voltage level of the charging voltage of the high voltage power supply that supplies voltage to the charging roller may become unstable. Particularly, in a color image forming apparatus configured such that a charging voltage supplied from a high voltage power source and applied to a charging roller is used in common by a plurality of image forming sections, the following phenomenon occurs. That is, a change in the load on the photosensitive drum in the image forming section where the pre-charge exposure by the pre-charge exposure device has been performed causes a change in the output of the charging voltage. Then, the charging voltage outputs applied to the charging rollers of the other image forming sections vary at the timings shown by Dy1, Dm1, Dc1, Dy2, Dm2, and Dc2 in FIG. 11. Here, Dy1, Dm1, Dc1, Dy2, Dm2, and Dc2 are the timings at which the light irradiation portion by pre-charging exposure on the photosensitive drum of each image forming section reaches the contact portion with the charging roller. As a result, the uniform potential state for image formation on the surface of the photosensitive drum (hereinafter referred to as background potential) shifts, causing problems such as density unevenness in images formed on the photosensitive drum of each image forming section. be.

The present invention was made under such circumstances, and an object of the present invention is to suppress fluctuations in charging voltage caused by changes in surface potential of a photosensitive drum due to pre-charging exposure.

In order to solve the above-mentioned problems, the present invention includes the following configuration.

(1) A first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, and a toner image that develops the electrostatic latent image formed on the first photosensitive drum. a first developing section that transfers the toner image on the first photosensitive drum to a transfer target, and a light emitting element, the toner image being transferred from the light emitting element to the first photosensitive drum. a first image forming section having a first static eliminator that removes static electricity from the surface of the first photosensitive drum by irradiating the emitted light; a second photosensitive drum; and a surface of the second photosensitive drum. a second charging member that charges the electrostatic latent image formed on the second photosensitive drum, a second developing section that develops the electrostatic latent image formed on the second photosensitive drum to form a toner image, and a toner on the second photosensitive drum. It has a second transfer unit that transfers an image to a transfer target, and a light emitting element, and irradiates the second photosensitive drum with light emitted from the light emitting element to remove static electricity from the surface of the second photosensitive drum. a second image forming section having a second static eliminator; a power supply unit that applies a charging voltage to the first charging member and the second charging member; , the second static eliminator, and the power supply unit applies the charging voltage to the second charging member when the charging voltage is applied to the first charging member. The control means changes the amount of light emitted from the light emitting element from the first amount of light to a second amount of light that is larger than the first amount of light, and changes the amount of light emitted from the second amount of light to the amount of light emitted by the light emitting element. An image forming apparatus characterized by controlling the amount of light to be changed to a third amount of light that is larger than the second amount of light.

(2) A first photosensitive drum, a first charging member that charges the surface of the first photosensitive drum, and a toner image that develops the electrostatic latent image formed on the first photosensitive drum. a first developing section that transfers the toner image on the first photosensitive drum to a transfer target, and a light emitting element, the toner image being transferred from the light emitting element to the first photosensitive drum. a first image forming section having a first static eliminator that removes static electricity from the surface of the first photosensitive drum by irradiating the emitted light; a second photosensitive drum; and a surface of the second photosensitive drum. a second charging member that charges the electrostatic latent image formed on the second photosensitive drum, a second developing section that develops the electrostatic latent image formed on the second photosensitive drum to form a toner image, and a toner on the second photosensitive drum. It has a second transfer unit that transfers an image to a transfer target, and a light emitting element, and irradiates the second photosensitive drum with light emitted from the light emitting element to remove static electricity from the surface of the second photosensitive drum. a second image forming section having a second static eliminator; a power supply unit that applies a charging voltage to the first charging member and the second charging member; , the second static eliminator, and the power supply unit applies the charging voltage to the second charging member when the charging voltage is applied to the first charging member. The control means changes the amount of light emitted from the light emitting element from the first amount of light to a second amount of light that is smaller than the first amount of light, and changes the amount of light emitted from the second amount of light to the amount of light emitted by the light emitting element. An image forming apparatus characterized by controlling the amount of light to be changed to a third amount of light that is smaller than the second amount of light.

According to the present invention, it is possible to suppress fluctuations in the charging voltage caused by changes in the surface potential of the photosensitive drum due to pre-charging exposure.

A sectional view showing a schematic configuration of an image forming apparatus in Examples 1 and 2 A diagram illustrating the schematic configuration of the pre-charging exposure device of Examples 1 and 2. A circuit diagram showing the circuit configuration of the light emission control circuit of the pre-charging exposure device of Examples 1 and 2. A diagram illustrating circuit characteristics of the light emission control circuits of Examples 1 and 2. A diagram illustrating the configuration of the high voltage power supply of the image forming apparatus of Examples 1 and 2. A diagram illustrating the outline of the cleanerless system of Examples 1 and 2. Timing chart showing the states of the pre-charging exposure device and photosensitive drum during printing operation in Example 1 A diagram explaining the photosensitive characteristics of the photosensitive drum of Example 2 Timing chart showing the states of the pre-charging exposure device and photosensitive drum during printing operation in Example 2 A diagram illustrating a light emission control method of the pre-charging exposure device of Example 2 Timing chart showing the status of the pre-charging exposure device and photosensitive drum during printing operation in a conventional example

Embodiments of the present invention will be described in detail below with reference to the drawings.

[Configuration of image forming apparatus]
FIG. 1 is a sectional view showing a schematic configuration of a color laser beam printer, which is an image forming apparatus to which the present invention is applied. The color laser beam printer 201 shown in FIG. 1 includes process cartridges 225y and 225m, which are image forming units that form images with toner colors of yellow (Y), magenta (M), cyan (C), and black (K). Equipped with 225c and 225k. Note that the configuration and operation of each process cartridge 225 are substantially the same. Furthermore, y, m, c, and k appended to the end of the codes of the members constituting the process cartridge 225 indicate toner colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Indicates that it is a member. Similarly, y, m, c, and k appended to the end of the code of components other than the process cartridge 225 indicate toner colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. This indicates that the member corresponds to the process cartridge. In the following, descriptions of y, m, c, and k added to the end of the reference numeral of a member will be omitted unless referring to a member of a specific process cartridge 225.

In the color laser beam printer 201 (hereinafter referred to as printer 201), each process cartridge 225 has a photosensitive drum 215 that is an image carrier. The photosensitive drum 215 is rotationally driven in the arrow direction (counterclockwise direction) in the figure by a drive source (not shown). A charging roller 216, a developing device 217, and a pre-charging exposure device 227 (details will be described later) are arranged around the photosensitive drum 215. A charging roller 216, which is a charging member, charges the surface of the photosensitive drum 215 to a uniform potential. Further, an electrostatic latent image is formed on the photosensitive drum 215 (on the photosensitive drum) by laser light irradiated from an optical device 210, which is an exposure section, which will be described later. A developing device 217 serving as a developing section develops the electrostatic latent image formed on the photosensitive drum 215 by a developing roller 229 by applying a developer (toner) to form a toner image. In this way, the process cartridge 225 is configured by integrating the photosensitive drum 215, the charging roller 216, the developing device 217, and the pre-charging exposure device 227.

In the printer 201, when the video controller 204 receives image data 203 including a print command and image information from the host computer 202, which is an external computer, it develops the image data and generates image data for forming an image. Based on the generated image data, the video controller 204 generates a video signal 205, which is data in a video signal format for controlling light emission of the laser diode 211 of the optical device 210, and outputs it to the engine controller 206.

The engine controller 206 has a CPU 209 and controls the image forming operation of the printer 201. Upon receiving the video signal 205 from the video controller 204, the CPU 209 serving as a control unit drives and controls a laser diode 211 disposed on a laser control board 208 in the optical device 210 to emit light in synchronization with the video signal 205. . A laser beam 212 emitted from a laser diode 211 corresponding to each process cartridge 225 is deflected by a rotating polygon mirror 207, passes through a lens 213, and is reflected by a folding mirror 214. The laser beam 212 reflected by the folding mirror 214 is irradiated onto the photosensitive drum 215 in the corresponding process cartridge 225.

The photosensitive drum 215 in each process cartridge 225 is charged to a desired amount of charge (potential) by a charging roller 216 . A laser beam 212 emitted from a laser diode 211 is irradiated to partially lower the surface potential of the photosensitive drum 215, thereby forming an electrostatic latent image on the surface of the photosensitive drum 215. Then, in order to visualize the electrostatic latent image formed by irradiation with the laser beam 212, the developing device 217 attaches developer (toner) on the developing roller 229 to the electrostatic latent image and places it on the photosensitive drum 215. to form a toner image corresponding to the electrostatic latent image.

The toner image formed on the photosensitive drum 215 is transferred onto the photosensitive drum 215 of each process cartridge 225 onto the intermediate transfer belt 219, which is a transfer target, by applying a primary transfer voltage to the primary transfer roller 218, which is a transfer section. The toner image formed on the image is transferred. The intermediate transfer belt 219 rotates in the direction of the arrow in the figure (clockwise direction). First, a toner image of yellow toner on the photosensitive drum 215y of the process cartridge 225y is transferred onto the intermediate transfer belt 219. Thereafter, the toner images formed on the photosensitive drums 215m, 215c, and 215k of the process cartridges 225m, 225c, and 225k with toner colors of magenta, cyan, and black are sequentially transferred to the intermediate transfer belt 219 to form a color image. Ru. When viewed from the rotational movement direction of the intermediate transfer belt 219, the process cartridges 225y, 225m, and 225c, which are the first image forming section, are arranged upstream of the process cartridge 225k, which is the second image forming section.

Further, as shown in FIG. 1, the printer 201 includes a cassette 220 containing recording material P, which is a recording medium. In synchronization with the image forming operation in the process cartridge 225 described above, the recording material P accommodated in the cassette 220 is fed to the conveyance path by the paper feed roller 222, and the conveyance roller disposed on the conveyance path feeds the recording material P. It is conveyed to the secondary transfer roller 226.

The secondary transfer roller 226 is pressed against an opposing roller via the intermediate transfer belt 219, and forms a secondary transfer portion 223 where the intermediate transfer belt 219 and the secondary transfer roller 226 are in contact with each other. A secondary transfer voltage is applied to the secondary transfer roller 226, and the toner image formed on the intermediate transfer belt 219 is transferred onto the recording material P at the secondary transfer section 223. Note that desired charging voltages, developing voltages, primary transfer voltages, and secondary transfer voltages are applied to the above-mentioned charging roller 216, developing roller 229, primary transfer roller 218, and secondary transfer roller 226, respectively. The CPU 209 performs control so that voltages corresponding to the characteristics of the recording material P are supplied from the high voltage power supply that supplies each applied voltage. Thereafter, the recording material P to which the toner image has been transferred is heated and pressurized in the fixing device 224, and the toner image is fixed to the recording material P. The recording material P to which the toner image has been fixed is discharged outside the printer 201. It is discharged to the department.

Furthermore, in each process cartridge 225, by irradiating the photosensitive drum 215 with light emitted from a light emitting element, the surface of the photosensitive drum 215 after the primary transfer is neutralized and the surface potential is smoothed. A pre-charging exposure device 227, which is a static eliminating section, is provided. As shown in FIG. 1, the pre-charging exposure device 227 is located downstream of the primary transfer roller 218 and upstream of the charging roller 216 in each process cartridge 225 with respect to the rotational direction of the photosensitive drum 215. It is located. That is, the pre-charging exposure device 227 is located downstream of the transfer section where the photosensitive drum 215 and the intermediate transfer belt 219 come into contact, and upstream of the charging section where the photosensitive drum and the charging roller 216 come into contact. This configuration exposes the surface of the photosensitive drum 215 on the side.

[Configuration of pre-charging exposure device]
Next, the configuration of the pre-charging exposure device 227 will be explained. Here, in order to simplify the explanation, only the configuration of the pre-charging exposure device 227y of the process cartridge 225y whose toner color is yellow will be explained. Note that the pre-charging exposure devices 227m, 227c, and 227k provided in the other process cartridges 225m, 225c, and 225k also have the same configuration as the process cartridge 225y.

FIG. 2 is a diagram illustrating the configuration of a pre-charging exposure device 227y that performs pre-charging exposure of the photosensitive drum 215y. As shown in FIG. 2, the pre-charging exposure device 227y includes a light emitting element 301y and a light guide 302y. The light emitting element 301y is a light emitting element used for pre-charging exposure, and is installed on the main body side of the printer 201. On the other hand, the light guide 302y is a light guide member for irradiating the photosensitive drum 215y with light emitted from the light emitting element 301y, and is installed in a cartridge tray (not shown) that holds the process cartridge 225y. The light guide 302y is arranged downstream of the primary transfer roller 218y shown in FIG. 1 in the rotational direction (counterclockwise direction) of the photosensitive drum 215, and upstream of the charging roller 216y.

As shown in FIG. 2, the light guide 302y is arranged approximately parallel to the axis (rotation axis) direction of the photosensitive drum 215y, and one longitudinal end of the light guide 302y receives light emitted from the light emitting element 301y. A light incident section 303y is provided. The amount of light emitted by the light emitting element 301y is controlled at a predetermined timing by a light emitted amount control unit (not shown). The light emitted from the light emitting element 301y and incident on the light guide 302y becomes diffused light from the side surface of the light guide 302y, and is irradiated onto the photosensitive drum 215y, thereby eliminating the surface potential of the photosensitive drum 215.

In this embodiment, the amount of light emitted by the pre-charging exposure device 227 is adjusted to a preset amount of light, but for example, a mechanism for adjusting the amount of light emitted may be provided. That is, a light receiving element that detects the amount of light emitted from the pre-charging exposure device 227 is arranged near the light emitting element 301, the light guide 302, and the photosensitive drum 215. A mechanism may be provided to adjust the amount of emitted light based on the amount of emitted light detected by the light receiving element, depending on deterioration of the light emitting element 301, dirt on the light guide 302, and change in light receiving sensitivity of the photosensitive drum 215. Further, although the configuration in which the light guide 302y is installed in a cartridge tray (not shown) has been described here, the following configuration may be used, for example. That is, a configuration in which the light guide 302y is arranged in the process cartridge 225y, a configuration in which an LED array is used instead of the light guide 302y, and a configuration in which the light guide 302y is eliminated to simplify the apparatus and light is directly irradiated onto the photosensitive drum 215. But that's fine.

[Control circuit of pre-charging exposure device]
Next, a circuit for controlling the amount of light emitted from the light emitting element 301 of the pre-charging exposure device 227 will be described. To simplify the explanation, only the circuit that controls the amount of light emitted from the light emitting element 301y of the pre-charging exposure device 227y of the process cartridge 225y whose toner color is yellow will be explained here. Note that the circuits that control the amount of light emitted from the light emitting elements 301m, 301c, and 301k of the other process cartridges 225m, 225c, and 225k have the same configuration as that of the process cartridge 225y.

FIG. 3 is a circuit diagram showing the circuit configuration of the light emission control circuit of the light emitting element 301y of the pre-charging exposure device 227y. The light emission control circuit includes a light emitting element 301y that is a light emitting diode (hereinafter also referred to as a light emitting diode 301y), resistors 401y and 404y, a capacitor 402y, and a transistor 403y. A PWM signal for controlling the amount of light emitted from the light emitting element 301y is output from the CPU 209 (FIG. 1) of the engine controller 206 to the light emission control circuit. The PWM signal is smoothed by an RC filter including a resistor 401y and a capacitor 402y, and is input to the base terminal of a transistor 403y. The voltage input to the base terminal of the transistor 403y is configured to be adjustable according to the onDuty of the PWM signal.

The cathode terminal of the light emitting diode 301y is connected to the collector terminal of the transistor 403y, and the anode terminal of the light emitting diode 301y is connected to the power supply voltage Vcc. On the other hand, the emitter terminal of the transistor 403y is connected to ground via a resistor 404y. A voltage dropped by the base-emitter voltage with respect to the base terminal voltage of the transistor 403y is applied to the resistor 404y. As a result, the current flowing through the light emitting element 301y is controlled, and the amount of light irradiated onto the photosensitive drum 215y changes depending on the value of the current flowing through the light emitting element 301y.

FIG. 4 shows the relationship between the OnDuty (ratio of on state in one cycle) of the PWM signal output from the CPU 209, the base voltage characteristics of the transistor 403y, the control current ratio flowing to the light emitting element 301y, and the light emission ratio of the light emitting element 301y. This is the graph shown. FIG. 4A is a graph showing the relationship between the OnDuty of the PWM signal and the base voltage input to the transistor 403y. In FIG. 4A, the horizontal axis indicates the OnDuty (unit: %) of the PWM signal, and the vertical axis indicates the base voltage (unit: V) of the transistor 403y. As shown in FIG. 4A, when the OnDuty of the PWM signal is 20%, the voltage at the base terminal of the transistor 403y is 0.7V, which is a voltage that turns on the transistor 403y. Furthermore, when the OnDuty of the PWM signal is 100%, the voltage at the base terminal of the transistor 403y is 3.3V.

FIG. 4B is a graph showing the relationship between the OnDuty of the PWM signal and the control current ratio flowing through the light emitting element 301y. The horizontal axis of FIG. 4B shows the OnDuty (unit: %) of the PWM signal, and the vertical axis shows the control current ratio (unit: %) flowing through the light emitting element 301y (light emitting element 301y). The control current ratio indicates the ratio of the current flowing through the light emitting element 301y when the current flowing through the light emitting element 301y when the onDuty of the PWM signal is 100% is 100%. When the OnDuty of the PWM signal exceeds about 20%, the transistor 403y turns on, current begins to flow to the light emitting element 301y, and the light emitting element 301y of the pre-charging exposure device 227y can be controlled to emit light from a low light amount range. It is shown that.

Further, FIG. 4(c) is a graph showing the relationship between the onDuty of the PWM signal and the light emission ratio of the light emitting element 301y. The horizontal axis of FIG. 4C shows the OnDuty (unit: %) of the PWM signal, and the vertical axis shows the light emission ratio (unit: %) of the light emitting element 301y. The emitted light amount ratio indicates the ratio of the emitted light amount of the light emitting element 301y when the emitted light amount when the onDuty of the PWM signal is 100% is set to 100%. In this way, the CPU 209 adjusts the amount of charge removed from the photosensitive drum 215y when performing pre-charging exposure by varying the onDuty of the output PWM signal and controlling the amount of light emitted by the light emitting element 301y used for pre-charging exposure. be able to.

In this embodiment, the base voltage of the transistor 403y is controlled via an RC filter composed of a resistor 401y and a capacitor 402y, the current flowing through the light emitting element 301y is controlled, and the amount of light emitted by the light emitting element 301y is adjusted. We explained how to do this. In this embodiment, the amount of light emitted is adjusted by controlling the current flowing through the light emitting element 301y. Good too.

[High voltage power supply configuration]
Next, the configuration of the high voltage power supply of the printer 201 of this embodiment will be explained. FIG. 5 is a schematic cross-sectional view illustrating the configuration of a high voltage power supply that supplies high voltage to the process cartridges 225y, 225m, 225c, 225k, etc. that perform image formation. FIG. 5 schematically shows which high-voltage power source supplies the voltage applied to the charging roller 216, developing roller 229, primary transfer roller 218, and secondary transfer roller 226 of each process cartridge 225. ing.

In FIG. 5, a voltage generation circuit 601, which is a power supply unit, generates a charging voltage Vc1 to charge each process cartridge 225y, 225m, and 225c with toner colors of yellow (Y), magenta (M), and cyan (C). It is supplied to charging rollers 216y, 216m, and 216c. In this embodiment, in order to downsize the power supply device, charging rollers 216y, 216m, and 216c, which are charging members of a plurality of process cartridges 225, are supplied with a charging voltage from a common voltage generation circuit 601 (first power supply). It supplies Vc1. When supplying power from the voltage generation circuit 601 to the charging rollers 216y, 216m, and 216c, different voltages are supplied to the charging rollers 216y, 216m, and 216c by connecting a resistor, Zener diode, etc. between each charging roller. The configuration may be such that That is, for example, if a charging voltage is to be applied to the charging roller 216y, the charging voltage may be applied to at least the charging roller 216m. In this case, it goes without saying that the charging roller 216c or the charging roller 216k can be used instead of the charging roller 216m.

Further, a voltage dividing circuit constituted by a resistor 603 and a Zener diode 604 divides the charging voltage Vc1 to generate a developing voltage Vd1. In the voltage dividing circuit, one end of the resistor 603 is connected to a terminal of the voltage generating circuit 601 that outputs the charged voltage Vc1. Further, the other end of the resistor 603 is connected to the anode terminal of the Zener diode 604 and the developing rollers 229y, 229m, and 229c of the process cartridges 225y, 225m, and 225c. The cathode terminal of the Zener diode 604 is connected to ground (grounded). The developing voltage Vd1 generated by the voltage dividing circuit is supplied to the developing rollers 229y, 229m, and 229c of the process cartridges 225y, 225m, and 225c.

On the other hand, the voltage generation circuit 602 (second power supply) generates a charging voltage Vc2 and supplies it to the charging roller 216k of the process cartridge 225k whose toner color is black (K). In this embodiment, a voltage generation circuit 602 is provided separately from the voltage generation circuit 601 described above in order to independently supply a charging voltage when printing a monochrome image. Also, in the voltage generation circuit 602, similarly to the voltage generation circuit 601, a voltage division circuit is provided that divides the charging voltage Vc2 to generate the developing voltage Vd2. The voltage dividing circuit includes a resistor 605 and a Zener diode 606. One end of the resistor 605 is connected to a terminal that outputs the charging voltage Vc2 of the voltage generation circuit 602, and the other end of the resistor 605 is connected to the anode terminal of the Zener diode 606 and the developing roller 229k of the process cartridge 225k. The cathode terminal of the Zener diode 606 is connected to ground (grounded). The developing voltage Vd2 generated by the voltage dividing circuit is supplied to the developing roller 229k of the process cartridge 225k.

The voltage generation circuits 601 and 602 each have a voltage detection circuit section (not shown) so that the charging voltage supplied to each charging roller 216 can be varied in accordance with the operating environment of the printer 201 and the aging of the photosensitive drum 215. There is. In this embodiment, the charging voltage Vc1 generated by the voltage generation circuit 601 is applied to the charging rollers 216y, 216m, and 216c, and is used to form a background potential on the surfaces of the photosensitive drums 215y, 215m, and 215c. On the other hand, the charging voltage Vc2 generated by the voltage generation circuit 602 is applied to the charging roller 216k and used to form a background potential on the surface of the photosensitive drum 215k. Note that the charging voltage Vc1 generated by the voltage generation circuit 601 and the charging voltage Vc2 generated by the voltage generation circuit 602 have the same output value. On the other hand, the developing voltage Vd1 is applied to the developing rollers 229y, 229m, and 229c, and the developing voltage Vd2 is applied to the developing roller 229k, and is used to attach toner to the electrostatic latent image formed on each photosensitive drum 215. Ru.

The voltage generation circuit 607 generates a primary transfer voltage, applies the primary transfer voltage to the primary transfer rollers 218y, 218m, 218c, and 218k, and transfers the toner image formed on the photosensitive drum 215 to the intermediate transfer belt 219. used for. Further, the voltage generation circuit 608 generates a secondary transfer voltage and applies the secondary transfer voltage to the secondary transfer roller 226 to transfer the toner image formed on the intermediate transfer belt 219 onto the recording material P. used.

[Effect of pre-charging exposure in cleanerless configuration]
Next, the cleanerless system will be explained using FIG. 6. Here, in order to simplify the explanation, the pre-charging exposure in the cleanerless configuration of the process cartridge 225y with yellow toner color will be explained. Note that the pre-charging exposure in the other process cartridges 225m, 225c, and 225k cleanerless configurations is the same as that for the process cartridge 225y.

FIG. 6 is a cross-sectional view showing the structure around the photosensitive drum 215y of the process cartridge 225y. In FIG. 6, L indicates the laser irradiation position of the laser beam 212y irradiated from the optical device 210 onto the photosensitive drum 215y. Further, D indicates a contact portion (contact position) between the photosensitive drum 215y and the developing roller 229y. Arrows in the developing roller 229y and the charging roller 216y indicate the rotation directions of the developing roller 229y and the charging roller 216y. Similarly, the arrow R1 indicates the rotation direction of the photosensitive drum 215y. Further, in the figure, black circles indicate toner of negative polarity, and white circles indicate toner of positive polarity. Note that in this embodiment, negative polarity is defined as normal polarity.

In the process cartridge 225y, in the transfer process, the toner image formed on the photosensitive drum 215y is transferred onto the intermediate transfer belt 219 by the primary transfer roller 218y. Transfer residual toner 701y that is not transferred onto the intermediate transfer belt 219 and remains on the surface of the photosensitive drum 215y is removed by the rotation of the photosensitive drum 215y in the R1 direction, and the toner 701y that is not transferred onto the intermediate transfer belt 219 is removed from the gap on the upstream side of the charging roller 216y in the rotational direction of the photosensitive drum. It is affected by the discharge in the portion 702y. The transfer residual toner 701y is negatively charged like the surface of the photosensitive drum 215y due to the influence of the discharge in the gap 702y. At the contact portion with the charging roller 216y, the surface potential of the photosensitive drum 215y is approximately -700V, and the potential difference of the charging roller 216y is approximately -1300V. Therefore, due to the potential difference, the transfer residual toner 703y on the negatively charged photosensitive drum 215y does not adhere to the surface of the charging roller 216y. As a result, the transfer residual toner 703y that did not adhere to the surface of the charging roller 216y passes through the charging roller 216y.

The negative transfer residual toner 703y on the photosensitive drum 215y that has passed the charging roller 216y moves in the R1 direction and reaches the laser irradiation position L where the laser beam 212y is irradiated. The transfer residual toner 703y is not large enough to block the laser beam 212y, so it does not affect the process of forming an electrostatic latent image on the surface of the photosensitive drum 215y.

Of the transfer residual toner 703y on the photosensitive drum 215y, the transfer residual toner 703y in a non-exposed area that has not been irradiated with the laser beam 212y at the laser irradiation position L is collected by the developing roller 229y by electrostatic force in the developing section D. Ru. On the other hand, the transfer residual toner 703y that has been irradiated with the laser beam 212y at the laser irradiation position L is not collected by the developing roller 229y due to electrostatic force, but continues to exist on the surface of the photosensitive drum 215y.

In this way, the transfer residual toner 701y remaining on the surface of the photosensitive drum 215y without being transferred to the intermediate transfer belt 219 is generally collected by the developing roller 229y. The transfer residual toner collected by the developing roller 229y is mixed with the toner inside the developing roller 229y and used again.

The cleanerless system of this embodiment includes a mechanism such as a cleaning blade that removes transfer residual toner that remains on the surface of the photosensitive drum 215y without being transferred to the intermediate transfer belt 219 during the transfer process in the primary transfer section. do not have. Therefore, it is necessary to prevent the transfer residual toner 701y remaining on the surface of the photosensitive drum 215y after the transfer process from adhering to the charging roller 216y. In order to charge the transfer residual toner 701y more negatively, the photosensitive drum 215y is neutralized by the pre-charging exposure device 227y, and the potential difference between the photosensitive drum 215y and the charging roller 216y is increased. This causes a stronger discharge to occur in the gap 702y formed near the charging roller contact portion. This strong discharge charges the transfer residual toner 701y more uniformly to negative polarity. Note that the potential of the exposed portion of the photosensitive drum 215y after static elimination by the pre-charging exposure is preferably about −150 V, which is approximately the same as the potential of the exposed portion by the laser beam 212y.

[Effect of pre-charging exposure on drum ghost]
Next, the effect of pre-charging exposure on drum ghosts will be explained. After the exposure process by the optical device 210, the development process by the developing roller 229, and the transfer process by the primary transfer roller 218, the surface potential of the photosensitive drum 215y becomes uneven depending on the image pattern formed on the photosensitive drum 215y. ing. If the surface potential of the photosensitive drum 215y is in such a state and the charging process is performed by the charging roller 216y in the next drum cycle, the surface of the photosensitive drum 215y may have a uniform potential depending on the image pattern formed in the previous cycle. It may not be possible to charge the battery. Therefore, in the next period of exposure process by the optical device 210, a desired electrostatic latent image cannot be formed on the photosensitive drum 215y, and a ghost image may occur. Therefore, after the transfer process by the primary transfer roller 218 and before the charging process by the charging roller 216y, light from the light emitting element 301y of the pre-charging exposure device 227y is irradiated onto the surface of the photosensitive drum 215y to increase the surface potential. to a predetermined residual potential level. This makes it possible to equalize the surface potential of the photosensitive drum 215y after the transfer process by the primary transfer roller 218, and to suppress the occurrence of drum ghosts.

[Light-on/off control of pre-charging exposure device]
The light emission control method of the pre-charging exposure device 227 in this embodiment will be explained using FIG. 7. FIG. 7 shows a timing chart showing the states of the pre-charging exposure device 227, the photosensitive drum 215, the charging voltage, etc. during the printing operation of the color laser beam printer 201 of this embodiment. The horizontal axis in FIG. 7 indicates time. In FIG. 7, 101y, 101m, 101c, and 101k are light emissions from the pre-charging exposure device 227 of each process cartridge 225 whose toner color is yellow (Y), magenta (M), cyan (C), and black (K), respectively. The amount of light emitted from the element 301 is shown. Further, 102y, 102m, 102c, and 102k indicate surface potential transitions in the vicinity of the position (pre-charging exposure area) of the photosensitive drum 215 that is irradiated with light from the light emitting element 301 of the pre-charging exposure device 227 of each process cartridge 225. It shows. Further, 103 indicates the output waveform of the charging voltage (-1300V) of the voltage generation circuit 601 described above. Further, 104y, 104m, 104c, and 104k indicate timings when the image forming area on the photosensitive drum 215 of each process cartridge 225 passes through the contact portion with the charging roller 216.

First, a method of controlling light emission of the pre-charging exposure device 227y in the process cartridge 225y, which is the yellow station, will be described. 101y and 102y in the figure indicate the amount of light emitted from the light emitting element 301y of the pre-charging exposure device 227y of the process cartridge 225y, respectively, and the amount of light emitted from the light emitting element 301y near the position of the photosensitive drum 215y (pre-charging exposure section) where light from the light emitting element 301y is irradiated. It shows the transition of surface potential. In this embodiment, the following control is performed when the photosensitive drum 215y is neutralized by light emitted from the light emitting element 301y of the pre-charging exposure device 227y after the transfer process. That is, as shown at 101y in the figure, the amount of emitted light from the light emitting element 301y of the pre-charging exposure device 227y is increased in eight steps for about 300 msec from the off state to reach a predetermined target. The amount of light emitted is controlled so that the amount of light is the same. Hereinafter, such control of the amount of emitted light will be referred to as stepwise ramp-up control of the amount of emitted light. In this way, by controlling the emitted light amount in stages, the surface potential of the photosensitive drum 215y in the vicinity of the pre-charging exposed area does not undergo a steep potential fluctuation, as shown at 102y in the figure, and the emitted light is emitted in stages. The voltage gradually changes from -300V to -150V depending on the amount of change in the amount of light. Similarly to the process cartridge 225y, for the process cartridges 225m, 225c, and 225k, the light emission amount of the light emitting element 301 of each pre-charging exposure device 227 is controlled in stages as shown at 101m, 101c, and 101k in the figure. conduct. By performing stepwise startup control of the light emitting element 301, the surface potential of each photosensitive drum 215 of the process cartridges 225m, 225c, and 225k is gradually changed from -300V to -150V as shown at 102m, 102c, and 102k in the figure. can be displaced to

Next, the charging voltage output supplied to each charging roller 216 of the process cartridges 225y, 225m, 225c, and 225k will be described. In the figure, 103 indicates the output waveform of the charging voltage Vc1 supplied from the voltage generation circuit 601, which is a high voltage power supply common to the charging rollers 216y, 216m, and 216c of the process cartridges 225y, 225m, and 225c. The charging voltage Vc1 is supplied to the charging rollers 216y, 216m, and 216c from a voltage generation circuit 601 that is a common high-voltage power source. Therefore, if the surface potential of the photosensitive drum 215 before and after the pre-charging exposure changes sharply, the output of the voltage generation circuit 601 will not be able to follow the load fluctuation caused by the steep surface potential fluctuation of the photosensitive drum 215, and the output of the charging voltage will fluctuate. Resulting in. Further, in the figure, Dy1 indicates the timing at which the pre-charging exposure portion on the photosensitive drum 215y irradiated with light from the light emitting element 301y of the process cartridge 225y reaches the contact portion with the charging roller 216y. Similarly, Dm1 indicates the timing at which the pre-charging exposure area on the photosensitive drum 215m irradiated with light from the light emitting element 301m of the process cartridge 225m reaches the contact area with the charging roller 216m. Further, Dc1 indicates the timing at which the pre-charging exposure portion on the photosensitive drum 215c irradiated with light from the light emitting element 301c of the process cartridge 225c reaches the contact portion with the charging roller 216c.

As shown at 102y, 102m, 102c, and 102k in the figure, the displacement of the surface potential at the position of the pre-charging exposure area where the above-mentioned pre-charging exposure is started on each photosensitive drum 215 of the process cartridges 225y, 225m, 225c, and 225k. is becoming more gradual. Therefore, at timings Dy1, Dm1, and Dc1 when the position of the pre-charging exposure section where the pre-charging exposure of the photosensitive drums 215y, 215m, and 215c has started reaches the charging roller 216, the voltage generation circuit 601 detects the load fluctuation caused by the pre-charging exposure. It is hardly affected by. As a result, the voltage generation circuit 601 can continue to supply the stable charging voltage Vc1 to each charging roller 216 of the process cartridges 225y, 225m, and 225c. Note that in the process cartridge 225y, a charging voltage is supplied to the charging roller 216k from a voltage generating circuit 602 that is different from the voltage generating circuit 601. Therefore, the configuration of the power supply device of this embodiment does not affect the charging voltage Vc1 supplied from the voltage generation circuit 601 to the process cartridges 225y, 225m, and 225c.

In the figure, 104y, 104m, 104c, and 104k indicate the timing at which the image forming area of each photosensitive drum 215 of the process cartridges 225y, 225m, 225c, and 225k passes through the contact portion with the charging roller 216. The image forming area on the photosensitive drum 215 is an area (section) where an electrostatic latent image is formed by the laser beam 212 irradiated from the optical device 210 in FIG. 6 described above. When the charging voltage Vc1 supplied from the voltage generating circuit 601 changes in the section where the image forming area on the photosensitive drum 215 contacts the charging roller 216, the background potential for image formation on the surface of the photosensitive drum 215 changes. As a result, density unevenness occurs in the formed image.

Further, as shown at 104y in the figure, timings Dm1 and Dc1 overlap during a period in which the image forming area on the photosensitive drum 215y of the process cartridge 225y passes through the contact portion with the charging roller 216y. Timings Dm1 and Dc1 indicate timings at which the pre-charging exposure portions of the photosensitive drums 215m and 215c of the process cartridges 225m and 225c come into contact with the charging rollers 216m and 216c. In this embodiment, as described above, stepwise startup control of each light emitting element 301 of the pre-charging exposure devices 227m and 227c of the process cartridges 225m and 225c is performed. Thereby, it is possible to suppress steep fluctuations in the surface potentials of the photosensitive drums 215m and 215c of the process cartridges 225m and 225c, and to suppress fluctuations in the output of the charging voltage Vc1 of the voltage generation circuit 601. Furthermore, in this embodiment, the timing to start the stepwise start-up control of each light emitting element 301 of the pre-charging exposure devices 227y, 227m, 227c of the process cartridges 225y, 225m, 225c is determined according to the timing at which image formation is started. It is shifted. Therefore, at timings Dy1, Dm1, and Dc1, a stable charging voltage Vc1 can be continuously supplied to each charging roller 216 of the process cartridges 225y, 225m, and 225c without steep fluctuations in the charging voltage output. As a result, the background potential in the image forming area of the photosensitive drum 215 of each process cartridge 225 does not fluctuate, making it possible to form a high quality image without density unevenness.

The light emission control method of the pre-charging exposure device 227 has been described above, but even when turning off the light emitting element 301 of the pre-charging exposure device 227, the light amount can be lowered stepwise from the target light amount to turn off the photosensitive drum 215. steep fluctuations in surface potential can be suppressed. That is, as shown at 101y in the figure, the light emission is performed so that the light emission amount of the light emitting element 301y of the pre-charging exposure device 227y is gradually decreased from a predetermined target light intensity state to an off state. The amount of light is controlled (hereinafter referred to as step-down control of the amount of emitted light). In this way, by controlling the emitted light amount to fall in stages, the surface potential of the photosensitive drum 215y in the vicinity of the pre-charging exposed area does not undergo a steep potential fluctuation, as shown at 102y in the figure, and the emitted light is emitted in stages. The voltage gradually changes from -150V to -300V depending on the amount of change in the amount of light. As a result, the occurrence of potential unevenness on the surface of the photosensitive drum 215 can be suppressed.

In this embodiment, the timing to start the step-down control of the light emitting elements 301 of the pre-charging exposure devices 227y, 227m, and 227c of the process cartridges 225y, 225m, and 225c is shifted based on the timing at which image formation ends. ing. As a result, when the light emitting element 301 is turned off, the output fluctuation of the charging voltage Vc1 at the timings Dy2, Dm2, and Dc2 at which the pre-charging exposed portions on the photosensitive drums 215 of the process cartridges 225y, 225m, and 225c come into contact with the charging roller 216 is suppressed. Can be suppressed. As a result, the voltage generation circuit 601 can continue to supply the stable charging voltage Vc1 to each charging roller 216 of the process cartridges 225y, 225m, and 225c, and can form high-quality images without density unevenness.

In this embodiment, a configuration has been described in which, in each process cartridge 225, the stepwise control of the amount of light emitted by the light emitting element 301 of the pre-charging exposure device 227 is performed. For example, when performing pre-charging exposure for a certain process cartridge 225, even if the light-emitting element 301 is abruptly raised from the off state to the target light intensity, local potential unevenness that occurs on the surface of the photosensitive drum 215 may cause the image to be formed outside the image forming area. may be formed. Specifically, in the configuration of this embodiment, light emission control for pre-charging exposure is performed for the photosensitive drum 215y of the process cartridge 225y, which is the yellow station, where the image formation start timing is earliest. In this case, since image formation is not affected, it is not necessarily necessary to perform stepwise control of the amount of light emitted by the light emitting element 301 of the pre-charging exposure device 227 described above. Similarly, when the light emitting element 301 of the pre-charging exposure device 227 is turned off, the position where local potential unevenness occurs on the surface of the photosensitive drum 215 may be formed outside the image forming area. Specifically, among the process cartridges 225y, 225m, and 225c to which the charging voltage Vc1 is supplied from the voltage generation circuit 601, the light extinguishing control for the pre-charging exposure is performed on the photosensitive drum 215c of the process cartridge 225c that completes image formation the slowest. This is the case. In this case, it is not necessarily necessary to perform stepwise reduction control of the amount of light emitted from the light emitting element 301 of the pre-charging exposure device 227.

Further, in the process cartridge 225k, a charging voltage is supplied to the charging roller 216k from a high voltage power supply (voltage generation circuit) different from that of the other process cartridges 225y, 225m, and 225c. Therefore, even if the output of the charging voltage supplied to the process cartridge 225k varies, it does not affect the background potential of each photosensitive drum 215 of the other process cartridges 225y, 225m, and 225c. Even in the case of such a configuration, it is not necessarily necessary to perform stepwise startup and shutdown control of the light emitting element 301 of the pre-charging exposure device 227.

Further, in this embodiment, the lighting/extinguishing control of the light emitting element 301 of the pre-charging exposure device 227 during the image forming operation has been explained. For example, in order to suppress potential unevenness that occurs on the surface of the photosensitive drum 215 due to pre-charging exposure, when controlling the light emitting elements 301 to turn on and off in the pre-charging exposure device 227 for purposes other than image formation, the light emitting elements The stepwise start-up control/shutdown control of 301 may be implemented.

As described above, in this embodiment, the amount of light emitted by the light emitting element 301 of the pre-charging exposure device 227 is controlled so that the amount of change in the surface potential on the photosensitive drum 215 before and after the pre-charging exposure is gentle. Thereby, it is possible to minimize output fluctuations in the charging voltage due to fluctuations in the surface potential of the photosensitive drum 215 (load fluctuations). As a result, when performing pre-charging exposure on the photosensitive drum 215 in order to suppress the adhesion of residual toner to the charging roller 216 and the occurrence of drum ghosts in a cleanerless system configuration, high quality images with uniform density can be obtained. image formation. In particular, in a color image forming apparatus configured to supply power to a plurality of charging members from a common power supply device, it is possible to form high-quality images without uneven image density.

As described above, according to this embodiment, it is possible to suppress fluctuations in the charging voltage caused by changes in the surface potential of the photosensitive drum due to pre-charging exposure.

Embodiment 2 In Embodiment 2, different from Embodiment 1, a stepwise ramp-up and ramp-down control of the amount of light emitted from a light emitting element of a pre-charging exposure device will be explained. The configurations of the image forming apparatus and the pre-charging exposure device of this example are the same as those of Example 1, and the same devices and members are denoted by the same reference numerals as those of Example 1, so that the description thereof will be omitted here.

[Photosensitive characteristics of photosensitive drum]
FIG. 8 is a diagram showing an example of an EV curve showing the photosensitive characteristics of each photosensitive drum 215 of the process cartridges 225y, 225m, 225c, and 225k. In FIG. 8, the horizontal axis represents the exposure amount (exposure light amount) E of the light emitting element 301 of the pre-charging exposure device 227, and the vertical axis represents the surface potential V of the photosensitive drum 215. In the EV curve in FIG. 8, when the surface potential (post-transfer potential) of the photosensitive drum 215 at the end of the primary transfer process is V1, when the light emitting element 301 of the pre-charging exposure device 227 irradiates the exposure light amount E2, the exposure The surface potential of the drum 215 (pre-charging, post-exposure potential) shifts to V2.

The EV curve shown in FIG. 8 shows that the surface potential of the photosensitive drum 215 is attenuated (lowered) by increasing the exposure amount E emitted by the light emitting element 301. Further, the high potential portion of the photosensitive drum 215 where the absolute value of the surface potential is large is an environment of a strong electric field, and recombination of charge carriers (electron-hole pairs) generated by exposure of the light emitting element 301 is difficult to occur. Therefore, even with a small exposure amount E, the surface potential of the photosensitive drum 215 is greatly attenuated. On the other hand, a low potential portion of the photosensitive drum 215 where the absolute value of the surface potential is small is a weak electric field environment, and charge carriers generated by exposure by the light emitting element 301 are likely to recombine. Therefore, even if the exposure amount E is large, the attenuation of the surface potential of the photosensitive drum 215 is small. That is, when the pre-charging exposure device 227 removes static electricity from the photosensitive drum 215, the surface potential of the photosensitive drum 215 has the following characteristics. The photosensitive drum 215 has a characteristic that in a low light intensity region where the amount of light emitted from the light emitting element 301 is lower than a predetermined light amount, the surface potential of the photosensitive drum 215 fluctuates greatly with respect to the amount of displacement of the amount of emitted light. Further, the photosensitive drum 215 has a characteristic that, in a high light intensity region where the amount of emitted light emitted from the light emitting element 301 is higher than a predetermined amount of light, the variation in the surface potential of the photosensitive drum 215 with respect to the amount of displacement of the amount of emitted light becomes small.

[Light-on/off control of pre-charging exposure device]
Next, a light emission control method of the pre-charging exposure device 227 in this embodiment will be explained. FIG. 9 shows a timing chart showing the states of the pre-charging exposure device 227, the photosensitive drum 215, the charging voltage, etc. during the printing operation of the color laser beam printer 201 of this embodiment. The horizontal axis in FIG. 9 indicates time. The timing chart of the amount of light emitted by the pre-charging exposure device 227, the surface potential of the photosensitive drum 215 near the pre-charging exposure section, the output waveform of the charging voltage, etc. shown on the vertical axis of FIG. 9 is the same as that of FIG. 7 described above. An explanation of how to read the figure will be omitted.

In this embodiment, when controlling the light emission of the light emitting element 301 of the pre-charging exposure device 227 of each process cartridge 225, the amount of emitted light 101 of the light emitting element 301 corresponds to the EV curve, which is the photosensitive characteristic of the photosensitive drum 215 described above. Controlled according to characteristics. Therefore, in this embodiment, stepwise start-up control is performed in which the light amount of the light emitting element 301 is gradually increased in a low light amount region, and the light amount of the light emitting element 301 is rapidly increased in a high light amount region. Thereby, as shown at 102y, 102m, 102c, and 102k in the figure, the surface potential of the photosensitive drum 215 can be neutralized without locally causing steep potential fluctuations. In the stepwise start-up control in Example 1, the light amount of the light emitting element 301 changes by a constant amount and is controlled linearly in proportion to time. On the other hand, in the staged start-up control of this embodiment, compared to the first embodiment, the timings Dy1, Dm1, when the pre-charging exposure portion on the photosensitive drum 215 of each process cartridge 225 reaches the contact portion with the charging roller 216, Fluctuations in the charging voltage at Dc1 can be further suppressed.

Also, in the step-down control for turning off the light emitting element 301 of the pre-charging exposure device 227, similarly to the light emission control of the light emitting element 301, the light emission is controlled according to the characteristics of the EV curve, which is the photosensitive characteristic of the photosensitive drum 215. The element 301 is controlled to turn off. In this embodiment, the amount of displacement of the surface potential of the photosensitive drum 215 is suppressed by performing step-down control in which the light amount of the light emitting element 301 is sharply reduced in a high light amount region and gradually reduced in a low light amount region. be able to.

FIG. 10 is a diagram illustrating an example of stepwise ramp-up control of the amount of light emitted from the light emitting element 301 described above. In FIG. 10, the horizontal axis represents time, and the vertical axis represents the amount of light emitted from the light emitting element 301 of the pre-charging exposure device 227. In the stepwise start-up control shown in FIG. 10, the amount of light emitted by the light emitting element 301 of the pre-charging exposure device 227 is increased in stages in a low light amount region where the photosensitive characteristics of the photosensitive drum 215 at the timing when light emission is started are more sensitive. On the other hand, in a high light amount region where the photosensitivity becomes dull, the amount of light emitted by the light emitting element 301 is sharply increased. By performing such stepwise start-up control, while suppressing fluctuations in the charging voltage due to static elimination in the pre-charging exposure device 227, the time required to ramp up the light emitting amount of the light emitting element 301 of the pre-charging exposure device 227 to the target light amount can be shortened. can do.

As explained above, the amount of light emitted by the light emitting element 301 of the pre-charging exposure device 227 is controlled according to the photosensitive characteristics of the photosensitive drum 215 so that the amount of change in the surface potential of the photosensitive drum 215 before and after the pre-charging exposure becomes gentle. do. Thereby, it is possible to further reduce the output fluctuation of the charging voltage due to the load fluctuation of the surface potential of the photosensitive drum 215. As a result, when performing pre-charging exposure on the photosensitive drum 215 in order to suppress adhesion of transfer residual toner to the charging roller 216 and drum ghost in a cleanerless system configuration, high-quality images with no image density unevenness can be obtained. Formation can be performed. In particular, in a color image forming apparatus having a configuration in which power is supplied to a plurality of charging members from a common power supply device, high-quality image formation without uneven image density can be performed.

As described above, according to this embodiment, it is possible to suppress fluctuations in the charging voltage caused by changes in the surface potential of the photosensitive drum due to pre-charging exposure.

209 CPU
215 Photosensitive drum 216 Charging roller 225 Process cartridge 227 Pre-charging exposure device 301 Light emitting elements 601, 602 Voltage generation circuit

Claims (18)

a first photosensitive drum; a first charging member that charges the surface of the first photosensitive drum; and a first charging member that develops an electrostatic latent image formed on the first photosensitive drum to form a toner image. 1 developing section, a first transfer section that transfers the toner image on the first photosensitive drum to a transfer object, and a light emitting element, and the light emitted from the light emitting element onto the first photosensitive drum. a first image forming unit having a first static eliminating unit that discharges static electricity from the surface of the first photosensitive drum by irradiating the first image forming unit;
a second photosensitive drum, a second charging member that charges the surface of the second photosensitive drum, and a second charging member that develops the electrostatic latent image formed on the second photosensitive drum to form a toner image. a second developing section, a second transfer section that transfers the toner image on the second photosensitive drum to a transfer target, and a light emitting element, and the light emitted from the light emitting element is directed to the second photosensitive drum. a second static eliminator that removes static electricity from the surface of the second photosensitive drum by irradiating the second image forming unit;
a power supply unit that applies a charging voltage to the first charging member and the second charging member;
A control means for controlling the first static eliminator and the second static eliminator,
Equipped with
The power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member,
The control means changes the amount of light emitted by the light emitting element from a first amount of light to a second amount of light that is larger than the first amount of light, and changes the amount of light from the second amount of light to the amount of light that is greater than the second amount of light. An image forming apparatus characterized in that the image forming apparatus controls the amount of light to be changed to a third amount of light that is larger. The first image forming section is arranged upstream of the second image forming section in the moving direction of the transfer target,
The image forming apparatus according to claim 1, wherein the control unit does not perform control to change the amount of light emitted by the light emitting element of the second static eliminator of the second image forming unit in a stepwise manner. . 3. The image forming apparatus according to claim 2, wherein the first light amount is a light amount when the light emitting element is in an off state. When the first photosensitive drum of the first image forming section is to be neutralized, the control means gradually increases the amount of light emitted from the light emitting element of the first static eliminating section from the off state to the target light amount. 4. The image forming apparatus according to claim 3, wherein the image forming apparatus increases the number of times. 5. The image forming apparatus according to claim 4, wherein the control means increases the amount of light emitted from the light emitting element by a constant amount of light. The control means may moderate the amount of light emitted in a low light amount region where the amount of light emitted by the light emitting element is lower than a predetermined light amount than in a high light amount region where the amount of light emitted by the light emitting element is higher than the predetermined light amount. The image forming apparatus according to claim 4, wherein the image forming apparatus increases the number of images. The first image forming section has a plurality of image forming sections,
7. The image forming apparatus according to claim 6, wherein the timing at which the first static eliminator starts eliminating static electricity in the plurality of image forming units differs depending on each of the image forming units. The first image forming section has a plurality of image forming sections,
7. The control means causes the first static eliminator to start static elimination in the plurality of image forming units in accordance with the timing at which image formation in each of the image forming units starts. image forming device. a first photosensitive drum; a first charging member that charges the surface of the first photosensitive drum; and a first charging member that develops an electrostatic latent image formed on the first photosensitive drum to form a toner image. 1 developing section, a first transfer section that transfers the toner image on the first photosensitive drum to a transfer object, and a light emitting element, and the light emitted from the light emitting element onto the first photosensitive drum. a first image forming unit having a first static eliminating unit that discharges static electricity from the surface of the first photosensitive drum by irradiating the first image forming unit;
a second photosensitive drum, a second charging member that charges the surface of the second photosensitive drum, and a second charging member that develops the electrostatic latent image formed on the second photosensitive drum to form a toner image. a second developing section, a second transfer section that transfers the toner image on the second photosensitive drum to a transfer target, and a light emitting element, and the light emitted from the light emitting element is directed to the second photosensitive drum. a second static eliminator that removes static electricity from the surface of the second photosensitive drum by irradiating the second image forming unit;
a power supply unit that applies a charging voltage to the first charging member and the second charging member;
A control means for controlling the first static eliminator and the second static eliminator,
Equipped with
The power supply unit is configured such that when the charging voltage is applied to the first charging member, the charging voltage is also applied to the second charging member,
The control means changes the amount of light emitted by the light emitting element from a first amount of light to a second amount of light that is smaller than the first amount of light, and changes the amount of light from the second amount of light to the amount of light that is smaller than the second amount of light. An image forming apparatus characterized in that the image forming apparatus controls the amount of light to be changed to a third amount of light that is smaller. The first image forming section is arranged upstream of the second image forming section in the moving direction of the transfer target,
The image forming apparatus according to claim 9, wherein the control means does not perform control so as to change the amount of light emitted by the light emitting element of the second static eliminator of the second image forming section in a stepwise manner. . The image forming apparatus according to claim 10, wherein the third light amount is a light amount when the light emitting element is in an off state. When terminating static elimination of the first photosensitive drum of the first image forming section, the control means gradually increases the amount of light emitted from the light emitting element of the first static eliminating section from the target light amount to the extinguished state. 12. The image forming apparatus according to claim 11, wherein the image forming apparatus reduces the number of images to . 13. The image forming apparatus according to claim 12, wherein the control means reduces the amount of light emitted from the light emitting element by a constant amount of light. The control means may moderate the amount of light emitted in a low light amount region where the amount of light emitted by the light emitting element is lower than a predetermined light amount than in a high light amount region where the amount of light emitted by the light emitting element is higher than the predetermined light amount. The image forming apparatus according to claim 12, wherein the image forming apparatus reduces the number of images. The first image forming section has a plurality of image forming sections,
15. The image forming apparatus according to claim 14, wherein the timing at which static elimination by the first static eliminating section in the plurality of image forming sections ends differs depending on each of the image forming sections. The first image forming section has a plurality of image forming sections,
15. The control unit is configured to terminate static elimination by the first static eliminator in the plurality of image forming units in accordance with a timing at which image formation in each of the image forming units ends. image forming device. The first static eliminator is located downstream of the first transfer unit in the rotational direction of the first photosensitive drum and upstream of the first charging member in the rotational direction of the first photosensitive drum. placed in
The second static eliminator is located downstream of the second transfer unit in the rotational direction of the second photosensitive drum and upstream of the second charging member in the rotational direction of the second photosensitive drum. The image forming apparatus according to any one of claims 1 to 16, characterized in that the image forming apparatus is arranged in a. Any one of claims 1 to 16, wherein the charging voltage applied to the first charging member and the charging voltage applied to the second charging member have the same output value. The image forming apparatus described in .

Images